I'll address your second question first. The SAT isn't easier or harder on certain test dates - that's just a rumor! You won't have a better chance of earning a high score if you take it at a particular time.
Now for the scoring. Your raw score on any given section of the SAT is pretty simple to understand. It's just the number of questions you answered correctly. For example, the total number of questions on the Math section is 58. This means that your raw score on the Math section would be a 49 if you answered 49 questions correctly and 9 questions incorrectly (or answered 5 incorrectly and left 4 blank; there is no difference between a blank question and an incorrect answer on the updated test). The raw scoring works the same way on Reading and Writing, although the scores on those two sections are ultimately combined to form your final Evidence Based Reading and Writing score out of 800, so it gets a little more complex later in the process.
For the Math section, your raw score is directly converted to a final score out of 800. This is done on a test to test basis, so a certain raw score may mean a slightly different final score depending on the curve that the College Board calculates. This is the confusing part because there's no absolute raw score to final score conversion - it's different every time.
For the Reading and Writing sections, raw scores are converted to scaled scores that range from 10 to 40 on each section. Again, this is done on a test to test basis, so it's not consistent every time. If you got a raw score of 40 on the Reading test (answered 40 out of 52 questions right), it could translate to around a 33 scaled score. If you also got a raw score of 40 on the Writing test (answered 40 out of 44 questions right), it could translate to a slightly higher scaled score of 36. Then, to get your final score out of 800, the two scaled scores are added together and multiplied by 10. In this case, the final score comes out to a 690.
Check out page 7 of this document for a scoring chart that might make the process a little clearer. It's difficult to understand because there is some ambiguity involved, but you can still roughly estimate your final score based on this conversion chart.